| A range of impurities（eg.,H2O,O2,SO2,NO2,H2S）from various CO2 capture processes may exist in supercritical CO2 transport pipeline in carbon capture and storage process,which are the potential corrosive factors to pose great risks to the durability of carbon steel pipelines during CO2 transport.For the supercritical CO2 fluids,the interaction between various impurities complicates the corrosion mechanism of the pipelines.To evaluate the corrosion degree and clarify the corrosion mechanism of supercritical CO2 transport pipeline will be conducive to establish the quality specifications of supercritical CO2 fluids and promote the application of carbon capture and storage.The corrosion behavior of X65 pipeline steel in supercritical CO2-H2O system containing O2,H2S,SO2 and/or NO2 impurities was investigated by means of thermodynamic analysis,weight loss measurements and surface characterization.The main conclusions are as follows:（1）The equation of state（EOS）was employed to calculate the mutual solubilities of CO2fluids.It was found that there existed a“blind area”in which cannot calculate the mutual solubilities of CO2 fluids over a specified temperature and pressure range.The“blind area”of SRK EOS disappeared at a temperature above 30oC,which was capable of calculating the mutual solubilities of supercritical CO2 fluids（The temperature and pressure are respectively over 31.1oC and 7.38 MPa）.The average absolute deviation of mutual solubilities calculated by SRK EOS was less than 5%,and had little fluctuation as the change of temperature.The thermodynamic calculation for the mutual solubilities of CO2-H2O system showed that when the pressure was near the critical pressure of CO2,the decline of temperature and pressure could cause a dramatic reduction of the solubility of H2O in CO2 fluids,and thus promoting the precipitation of water phase.The thermodynamic calculation for the mutual solubilities of CO2-H2O-H2S system suggested that the presence of H2S impurity could lead to the precipitation of water phase through reducing the solubility of H2O in CO2 fluids.The quantity of the water precipitation increased with the rising of H2S concentration.（2）The corrosion impact factor（Ci）was defined to evaluate the influence level of impurities on the corrosion of X65 pipeline steel in water-saturated supercritical CO2 system,and its expression was Ci=（Vi-V0）/V0.The results of Ci showed that NO2 had the greatest influence on the corrosion of X65 steel,followed by SO2 and H2S,and the influence of O2was the smallest among them,which was consistent with the thermodynamic analysis of water chemistry characteristics.Low concentrations of impurities could notably change the corrosion reaction mechanism via changing the water chemistry characteristics.The impurity could participate in the corrosion reaction and control the corrosion of X65 steel in varying degrees,and thus the corrosion effect of CO2 weakened.O2 or H2S with CO2 simultaneously controlled the corrosion process of X65 steel in water-saturated supercritical CO2-H2O-O2 or CO2-H2O-H2S systems.While SO2 or NO2 could inhibit the corrosion effect of CO2,the corrosion process was governed by SO2 or NO2 in water-saturated supercritical CO2-H2O-SO2or CO2-H2O-NO2 systems.（3）The synergistic interaction impact factor（Sm）was proposed to evaluate the synergistic effect of multiple impurities on the corrosion of X65 pipeline steel in water-saturated supercritical CO2 system,and its expression was Sm=Cm-∑Cs.The results of Sm showed that there existed a significant synergistic effect between various impurities.The essence of the synergistic effect was that the interactions between various impurities promoted the formation of H2SO4,S and/or H2O,which further aggravated the corrosion of X65 steel.The difference of Sm was determined by the products of the synergistic effect:The interaction between O2 and H2S promoted the precipitation of water phase and the formation of elemental sulfur.The interaction between O2 and SO2 promoted the formation of sulfuric acid.The interaction between H2S and SO2 prompted the formation of water phase and elemental sulfur,which further caused the formation of sulfuric acid due to the hydrolysis of elemental sulfur.The above complicated synergistic effects among O2,H2S and SO2 had the highest synergistic interaction impact factor corresponding to the highest corrosion rate.（4）In water-saturated supercritical CO2 system containing O2,SO2 or H2S impurities,the impurities and the interactions between various impurities caused the homogeneous precipitation of water phase in the form of thin water layer,which resulted in the general corrosion of X65 steel.While in the system containing NO2 with or without other impurities,NO2 and the interactions between NO2 and other impurities caused the inhomogeneous precipitation of water phase in the form of water droplet accumulation,which initiated the localized corrosion of X65 steel.There was a rapid initiation of corrosion pit at the initial corrosion stage in water-saturated supercritical CO2-NO2-O2-SO2-H2S system.The propagation rate of the corrosion pits greatly reduced in depth direction with corrosion time,and the corrosion morphology of X65 steel shifted from localized corrosion towards general corrosion after a long-term corrosion.The impurity interactions preferentially occurred to form FeSO4 and S in the early corrosion stage.As the corrosion time prolonged,S disappeared in the corrosion scale via participating in the subsequent corrosion reaction,and（NH4）Fe3（SO4）2（OH）6 and FeOOH formed due to the subsequent reactions of the impurity interaction products.（5）The critical content of water,with the occurrence of turning point of the corrosion rate and the characteristics of corrosion scale,was 1500 ppmv in supercritical CO2 system containing 200 ppmv O2,200 ppmv SO2 and 200 ppmv H2S at 10 MPa and 50oC.The corrosion reaction mechanism of X65 steel changed with the water content in this system.When the water content was lower than 1500 ppmv,the corrosion of X65 steel was mainly controlled by the synergistic effect of impurities.However,the synergistic effect of impurities and the corrosion effect of each impurity simultaneously controlled the corrosion of X65 steel when the water content was above 1500 ppmv. |
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